1. Field of the Invention
This invention relates to a process and apparatus for adjusting a semiconductor wafer by electron beam illumination.
2. Description of the Prior Art
In the production of semiconductor components, photolithographic etching techniques are used to etch structures in semiconductor wafers or in layers arranged on the semiconductor wafers. Irradiation masks are optically aligned on light-sensitive lacquer which is arranged on the semiconductor wafer. In addition to light-optical projection and copying processes, electron-optical processes are also used. In comparison to the light-optical projection process, the electron-optical processes have the advantage that projection inaccuracies caused by diffraction phenomena are less. When electron-optical projection and copying processes are used, the semiconductor wafers provided with an electron-sensitive lacquer must be adjusted in relation to the irradiation mask. In the case of electron beam alignment, in which an electron beam deflected by electromagnetic means automatically produces an illuminated area on a radiation-sensitive lacquer layer, the semiconductor wafer must be adjusted with respect to a radiation reference which is struck by the non-deflected beam. This adjustment must be highly accurate, particularly when the semiconductor wafer is provided with existing structures, which, in the production of a semiconductor device, must have a definite position in relation to the subsequently applied structures.
In accordance with the prior art, the adjustment with the aid of an electron beam is effected by moving the beam across an adjustment mark in the form of a stage arranged on the semiconductor wafer. When the electron beam penetrates into the material comprising the adjustment mark, it releases secondary or backscatter electrons which move away from the adjustment mark in a direction which is different to the direction of the oncoming electron beam. The secondary electrons and the backscatter electrons are intercepted by a detector. For purposes of adjustment, either the semiconductor wafer or the electron beam source is displaced until the detector supplies the desired signal. When the semiconductor wafer and the adjustment mark in the form of a stage on the semiconductor wafer are coated with an electron sensitive lacquer, as is necessary when an electron-optical illuminating process is being used, the signal produced by the backscatter and secondary electrons is considerably flattened and blurred. This results since the secondary and backscatter electrons are subject to additional scatter processes in the overlying electronsensitive lacquer. Blurred and unclear signals of this type are not very suitable, particularly for automatic adjustment. Also, a further complication arises since for secondary and backscatter electrons, the signal to noise ratio is small.